The present invention relates to a liquid crystal display apparatus such as a liquid crystal projector and a liquid crystal television.
The above-mentioned liquid crystal display apparatus uses a liquid crystal modulation element for converting light from a light source into images.
Some of the liquid crystal modulation elements are realized by putting nematic liquid crystal having positive dielectric anisotropy between a first transparent substrate having a transparent electrode (common electrode) formed thereon and a second transparent substrate having a transparent electrode (pixel electrode) forming pixels, wiring, switching elements and the like formed thereon. The liquid crystal modulation element is referred to as a Twisted Nematic (TN) liquid crystal modulation element in which the major axes of liquid crystal molecules are twisted by 90 degrees continuously between the two glass substrates. This liquid crystal modulation element is used as a transmissive liquid crystal modulation element.
Some of the liquid crystal modulation elements utilize a circuit substrate having reflecting mirrors, wiring, switching elements and the like formed thereon instead of the abovementioned second transparent substrate. This is called a Vertical Arrangement Nematic (VAN) liquid crystal modulation element in which the major axes of liquid crystal molecules are oriented in homeotropic alignment substantially perpendicularly to two substrates. The liquid crystal modulation element is used as a reflective liquid crystal modulation element.
In these liquid crystal modulation elements, typically, Electrically Controlled Birefringence (ECB) effect is used to provide retardation for a light wave passing through a liquid crystal layer to control the change of polarization of the light wave, thereby forming an image from the light.
In the liquid crystal modulation element which utilizes the ECB effect to modulate the light intensity, application of an electric field to the liquid crystal layer moves ionic materials present in the liquid crystal layer. When a DC electric field is continuously applied to the liquid crystal layer, the ionic materials are pulled toward one of two opposite electrodes. Even when a constant voltage is applied to the electrodes, part of the electric field applied to the liquid crystal layer is cancelled out by the charged ions to substantially attenuate the electric field applied to the liquid crystal layer.
To avoid such a phenomenon, a line inversion drive method is typically employed in which the polarity of an applied electric field is reversed between positive and negative for each line of arranged pixels and is changed in a predetermined cycle such as 60 Hz or the like. In addition, a field inversion drive method is used in which the polarity of an applied electric field to all of arranged pixels is reversed between positive and negative in a predetermined cycle. Those drive methods can avoid the application of the electric field of only one polarity to the liquid crystal layer to prevent the unbalanced ions.
This corresponds to controlling the effective electric field applied to the liquid crystal layer such that it always has the same value as the voltage applied to the electrodes.
The variations of the effective electric field applied to the liquid crystal layer, however, are caused not only by the abovementioned movement of the ionic materials but also by other factors. One of the other factors causes trapping of charge of electrons or holes in a non-conductive film such as a liquid crystal alignment film made of an insulating material, a reflection enhancing film, and an inorganic passivation film for preventing dissolution of metal. The trapping causes charge-up on the interface of the film, and that electrostatic charge changes the effective electric filed applied to the liquid crystal layer with time.
The charging phenomenon may be seen due to the shape in the transmissive liquid crystal modulation element and occurs prominently in the reflective liquid crystal modulation element including opposite electrodes formed of different materials (mirror metal and indium tin oxide (ITO) film).
The probability of excitation of electrons or holes varies depending on the amounts of light energy and photon energy applied to the liquid crystal modulation element. In an irradiation time period from start of lighting of a lamp in a liquid crystal display apparatus (start of light irradiation), the charge in the interface layer of the liquid crystal is gradually accumulated to produce a potential difference between the opposite mirror electrode and ITO transparent electrode, which shifts the optimal potential difference between the opposite electrodes. As a result, as the light irradiation time elapses or the intensity of light irradiation is increased, flicker is more noticeable in a displayed image.
Japanese Patent Laid-Open No. 2002-365655 and Japanese Patent Laid-Open No. 2005-49817 have disclosed methods in which a potential difference between opposite electrodes is adjusted to the optimal level to minimize flicker. Particularly, in the method disclosed in Japanese Patent Laid-Open No. 2005-49817, a work-function adjusting film layer is formed on a reflecting pixel electrode to control the work function of the reflecting electrode to be ±2% or less relative to the work function of a transparent electrode (ITO film electrode) opposite thereto, thereby reducing charge-up on an interface layer of the liquid crystal to avoid occurrence of flicker or image sticking.
A typical liquid crystal display apparatus has a plurality of display modes, such as a standard mode, a cinema mode, and a contrast mode, in which a light source provides various levels of brightness, an optical filter is inserted into or removed from the optical path from the light source to the liquid crystal modulation element, an aperture stop is provided or not in the optical path from the light source, or the aperture stop provides various diameters of its aperture opening.
In the respective display modes, the amount of light energy applied to the liquid crystal modulation element is varied to change the amount of charge-up on the interface layer of the liquid crystal to change the potential difference between the opposite mirror electrode and ITO transparent electrode. In other words, the optimal potential difference between the opposite electrodes depends on the display mode. If the same potential difference is used as the optimal potential difference between the opposite electrodes regardless of the display mode, the occurrence of flicker cannot be prevented in some of the display modes.
If switching between the display modes produces a potential difference between the opposite mirror electrode and ITO transparent electrode, an additional problem occurs. Specifically, the constant DC electric field is continuously applied to the liquid crystal layer, so that ionic materials present in a small amount in the liquid crystal layer is pulled toward one of the opposite electrodes. The ionic material may be pulled toward the interfaces on both sides of the liquid crystal layer depending on the polarity of the charge of the ion.
Since the ions attached to the interface of the electrode are moved in accordance with the amplitude of a drive potential in the field inversion drive, the attachment state of the ions varies with the level of the amplitude of the drive potential. This results in variations of the effective electric field applied to the liquid crystal layer at different positions in a display area, which causes sticking. When the same image is displayed for a long time and then a different image is displayed, the previous image is seen as an afterimage. This is called the image sticking (or simply, sticking).
The control methods disclosed in Japanese Patent Laid-Open No. 2002-365655 and Japanese Patent Laid-Open No. 2005-49817, however, do not make control suitable for various display modes. Thus, switching between the display modes may lead to the inability to prevent flicker or the occurrence of the sticking.